Shot cycle Dynamics in 3 Spring-Piston Airguns Chap 7

Note from the Publisher:

Please be so kind as to note the corrections made on July 11, 2021.
Corrections have been inserted in Bold Face.
The chart in fig 7.5 has also been corrected to the proper values.
We apologize for the compound mix-ups and acknowledge the help provided by Britt Salter in identifying the issue.

​Does a higher energy spring decrease accuracy in a springer air rifle?

​In this chapter we explore what happens when higher power springs are used in my FWB 124 and LGU air rifles. I’ve heard that these rifles are optimized for 12 ft-lbs, but they did quite well with higher energy springs. Let’s start with the FWB 124. When I lubricated my FWB 124 with Krytox, the muzzle velocity dropped dramatically, so I decided to put an older, higher power spring to get the muzzle velocity back up. This gave me the opportunity to test how the FWB 124 performs with lower and higher energy springs. The lower energy spring was a Maccari Slightly Softer spring, which produced muzzle energies around 11.2 ft-lb. The higher energy spring was a Maccari Pro-Mac, which produced muzzle energies around 13.3 ft-lb.  I would expect that the weaker spring would result in more docile recoil and therefore better accuracy. Figure 7.1 supports this assumption, with the average of eight 10-shot groups for the weaker spring in Fig. 7.1a) being a bit smaller (although within the error bars) of the average of eight 10-shot groups for the stronger spring in Fig. 7.1b).
​The standard deviation and extreme spread of the muzzle velocities were pretty much the same for both springs. 

Fig. 7.1 FWB 124 10-shot groups off bench at 20 yards with a) Maccari Slightly Softer mainspring (red rectangle) and b) Maccari Pro-Mac mainspring.

​A 2 ft-lb (19%) increase in muzzle energy is significant, so I was curious to see how that increase would affect the recoil traces. I looked at the recoil of the FWB 124 with the two springs over longer times in Fig. 7.2a) and shorter times in Fig. 7.2b). Figure 7.2b) also shows when the pellets exited the muzzle. Surprisingly, the position, velocity, and acceleration traces are nearly identical over the first 0.015s! The main difference is that the later oscillations are shifted slightly to the right (later times) for the weaker Slightly Softer spring (purple traces). These occur well after the pellet has left the barrel, so I doubt that they had much effect on the accuracy or muzzle velocity. Maybe they are just due to the different springs sloshing around differently once the pellet has left the barrel?

Fig. 7.2 FWB 124 recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle for the Maccari Slightly Softer and the Maccari Pro-Mac mainsprings. a) looks at longer times and b) focuses at shorter times and shows the pellets leaving the barrel (vertical red lines).

​Now let’s take a look at my LGU. For the past year, I’ve been the only competitor in the World Field Target piston class at the monthly Rochester Brooks matches (https://www.rbgunclub.com/field-target/), so I decided to try the hunter field target piston class. HFT has a muzzle energy limit of 20 ft lbs, so I replaced the 12 FT-LB (16 J) factory spring with a 20 J (15 ft-lb) spring that came with the rifle. The higher muzzle energy produces a flatter trajectory that should really help with accuracy and ranging at longer distances. 
Figure 7.3a) shows some targets at 20 yards off the bench with both springs and Fig. 7.3b) shows three targets at 52 yards off the bench with the 20 J spring. At 20 yards, the accuracy was pretty much the same with both springs despite the fact that recoil was significantly stronger with the 20 J spring, which increased the muzzle energy from 11.1 ft-lb to 14.4 ft-lb.
The variation in muzzle velocity was similar for both springs. I was very excited by the accuracy that the 20 J spring produced at 52 yards. The three 10-shot groups at 52 yards are the best I’ve ever shot with a spring piston air rifle past 50 yards. I was especially pleased that the groups drifted only slightly, with the third group back on top of the first group. All the groups had ctc distances under 0.9”, with the first and third groups under 0.75”. Remember, these are 10-shot groups!
​The higher muzzle velocity also took off about an inch of pellet drop at 52 yards, which will help reduce misses due to ranging errors. For comparison with 10-shot groups at 52 yards using the 16 J spring, please look at Fig. 5.6 in Ch. 5.

Fig. 7.3 LGU 10-shot groups off bench at a) 20 yards with 16J (top) and 20J (bottom, red rectangle) mainspring and b) at 52 yards with 20J mainspring.

​Figure 7.4 shows the recoil traces from my LGU with the 16 J and 20 J springs. Unlike the FWB 124, where increasing the muzzle energy by a couple of ft-lbs didn’t make much difference in the recoil traces, with the LGU a similar increase in muzzle energy made a big difference in the recoil. I aligned the 16 J and 20 J traces using the pellet exit signals (bottom curves in middle plot). The amplitudes of the velocity and acceleration peaks and dips were clearly bigger with 20 J spring. These peaks and dips also were shifted to longer times, suggesting that the shot cycle took a bit longer with the 20 J spring. This is opposite to the behavior in the FWB 124, where the weaker spring shifted the recoil traces to slightly longer times (to the right). Ideally, the traces should be aligned according to when the piston was released, but this is hard to nail down very precisely, so I had to use the pellet exit time, which will be slightly different due to differences in muzzle velocity.

Fig. 7.4 LGU recoil traces showing the position, velocity, and acceleration of the sled-mounted rifle for the factory 16 J and 20 J mainsprings. Vertical red lines show the when pellets leave the barrel. Note that for the 20 J spring only a single light gate was used, so there’s only one pulse in the bottom orange trace in the middle graph.

​Finally, I checked to see how the efficiency changes when going to a more powerful spring.
Before discussing the efficiencies, I'd like to thank Britt Salter for catching some important errors in the original version of Fig. 7.5. Thanks Britt for catching the inconsistencies between the posted muzzle velocities and calculated efficiencies in the original version of this chapter!
Figure 7.5 has been corrected and shows that the stronger spring required more work to cock for both the FWB 124 and LGU, but for the FWB 124 the resultant increase in muzzle energy actually increased the efficiency, while the stronger spring in the LGU decreased efficiency. One difference with the FWB 124 is that the weaker spring was lubricated with moly/Superlube and the stronger spring was lubricated with Krytox, so the efficiency comparison is not as reliable as with the LGU, where both springs were lubricated with Krytox. The increase in efficiency with the stronger spring in the FWB 124 isn’t very dramatic, but at least one can conclude that efficiency in this case did not go down when a stronger spring increased the muzzle energy by 19%. I’m willing to live with the decrease in efficiency of the LGU with the stronger spring if it can maintain better accuracy at longer distances.

So for my FWB 124, increasing the muzzle energy from 11.2 ft-lb to 13.3 ft-lb may have hurt accuracy slightly at 20 yards, but not significantly, without affecting the standard deviation nor the extreme spread in the muzzle velocity. The shot cycle at short times was pretty much the same for these two springs, but it looks like the oscillations with the weaker spring were slightly delayed well after the pellet exited the barrel. The efficiency with the stronger spring was slightly better. It would really help to check the accuracy at longer distance to see if the higher muzzle velocity produced by the stronger spring can result in better accuracy at longer distances. The higher muzzle velocity of the stronger string will produce a flatter trajectory, which also will help at longer distances.
​For those of you who want to maximize the muzzle velocity of your FWB 124, I suggest that you try the factory piston seal. I was typically getting up to 50 fps greater muzzle velocity with the factory piston seal over any aftermarket seal that I tried using a variety of mainsprings.
For the LGU, the stronger spring decreased efficiency a bit and didn’t change accuracy at 20 yards. However, at 52 yards the accuracy looks very promising (personal best!) and the flatter trajectory will certainly help ranging and hitting targets at longer distances. The recoil felt stronger, which is backed up by the recoil traces, but that didn’t seem to hurt accuracy. If it weren’t for the 12 ft-lb limit in the World Field Target piston class, I would definitely use the stronger 20 J spring in those matches. Since the muzzle energy limit in the American Airgun Field Target Association’s Hunter piston division is 20 ft-lbs, I’m looking forward to trying the stronger spring in my LGU for hunter FT matches. I wonder if there’s anything that can get me a few more foot pounds out of the LGU?! It also would be interesting to see if the efficiency continues to drop in the LGU with even stronger springs. We expect that the shorter, central transfer port in the LGU should work better at higher power compared to the longer, offset transfer port in the FWB 124, but at least for this particular test, the FWB 124 did a bit better in terms of efficiency at higher power.